1. Technical Field
The invention relates to in situ estimation of the state of charge of a motor vehicle battery, particularly a lead acid battery.
2. Description of the Problem
Lead acid batteries are the conventional source for power used by automatic starters to crank start internal combustion engines installed on motor vehicles. Lead acid batteries also provide auxiliary power for other electrical components installed on such vehicles. Failure of a battery to supply power for starting can necessitate jump starting the engine or an expensive and time consuming call to service for assistance. It would be an advantage to operators to receive warning of impending battery failure in time to take corrective action before failure of a battery in the field.
The lead-acid batteries typically used in vehicles are rated according to the Society of Automotive Engineers SAE J537 specification. The J537 specification defines two different ways in which capacity is measured, Cold Cranking Amps (CCA), and Reserve Capacity (RC). CCA is an indication of a battery's ability to deliver high power for a short duration (the amperage that a fully charged battery it expected to deliver for 30 sec.). RC is an indication of total energy capacity (the number of minutes that a battery can deliver 25 amps). For example, a battery rated at 650 CCA is expected to delivered 650 amps for 30 sec. (under the controlled conditions set forth in the specification). Likewise, a battery rated at 180 RC is expected to deliver 25 amps of current for 180 minutes.
Lead acid batteries are constructed from closely spaced, alternating plates of sponge lead (Pb), which serve as the negative plates, and lead dioxide (PbO2), which serve as the positive plates. The plates are preferably substantially immersed in a sulfuric acid (H2SO4) water solution, which serves as an electrolyte. During discharge of the battery both plates react with the electrolyte and lead sulfate (PbSO4) forms on both the negative and positive plates. The concentration of acid in the electrolyte decreases. As the plates become more chemically similar and the acid strength of the electrolyte falls, a battery's voltage will begin to fall. From fully charged to fully discharged each cell loses about 0.2 volts in potential (from about 2.1 volts to 1.9 volts). The rate at which the reaction occurs governs energy flow and battery power characteristics. Many factors control the reaction rate, such as the amount of active material in the plates and the availability of the acid. When a battery discharges, the acid in the pores of the lead plates react first. The depleted electrolyte at the plates is replenished by the electrolyte in the rest of the battery. A lead acid battery thus can be viewed as having multiple reservoirs of available energy. One that is available for immediate use, the primary reservoir, and secondary reservoirs that replenish the primary. The physical integrity of the plates and the purity and concentration of the electrolyte determine the battery's total potential.
Optimally, recharging a battery would reverse the process of discharge, strengthening the acid in the electrolyte and restoring the original chemical makeup and physical structure of the plates. In practice however, the chemical reactions and resulting physical changes that produce current during discharge are not perfectly reversible. The reasons for this are several. For example, input and output currents are not symmetric. A motor vehicle battery can discharge several hundred ampere-seconds during the relatively brief period of cranking of an engine. Recharging then occurs during the first few minutes after the engine begins running at far lower rates of current flow. The cycle of repeated discharge and subsequent recharge of lead acid batteries results in chemical imbalances in and loss of the electrolyte solution, the formation of undesirable compounds on battery plates and physical deterioration of the plates.
Recharging a battery has various secondary effects, including polarization of the battery, overheating and the electrolytic decomposition of the water into molecular hydrogen and oxygen. These factors contribute to the battery not returning to its original state. Electrolysis of the water in the electrolyte reduces the physical volume, and quantity, of the electrolyte. Electrolytic breakdown of the water leaves the electrolyte excessively acidic, with consequential degradation of the battery plates. High temperatures developed during recharging can promote sulfation of the battery plates (i.e. the formation of hardened, relatively insoluble crystalline lead sulfate on the surface of the plates), which in turn increases a battery's internal resistance. To some extent sulfation and other factors resulting in the slow reduction of a lead acid battery's charge capacity can be controlled by avoiding overcharging, or by avoiding overheating of the battery stemming from excessively fast recharging, but in practice the slow deterioration of a battery is unavoidable.
Polarization results in a poorly mixed electrolyte and a condition where battery voltage reflects a full 2.1 volts per cell, but only because local areas of the electrolyte contain over concentrations of acid, which in turn can damage the plates.
As the physical condition of a battery deteriorates, its capacity to hold a charge, in terms of ampere-hours declines. This is the case even though the battery continues to exhibit a 2.1 volt potential per cell when charged to maximum. Accordingly, battery state of charge and available battery cranking power are not, over the long term, accurately reflected by open circuit voltage.
Battery condition is best indicated by the specific gravity of the battery's electrolytic solution. Conventionally, the best way to gauge the state of charge of a lead acid battery has been to measure the specific gravity of the electrolyte of a properly filled (and exercised) battery using a temperature compensated hydrometer. A load test of the battery under controlled conditions may be used, either in conjunction with a check of specific gravity or independently. A load test subjects a fully charged battery to an ampere load equal to ½ the rated cold cranking capacity of the battery (at −18 degrees Celsius) for 15 seconds, then measures the voltage and the current under load and requires referral to a voltage chart to assess battery condition. See page 48, Storage Battery Technical Service Manual, Tenth Edition, published by the Battery Council International, Chicago, Ill. (1987). Such procedures are obviously not easily practiced in the field, where driver/operators of vehicles could make use of a quick indication if a battery has sufficient cranking power to start an engine.
To meet the need for battery condition evaluation in the field and to provide an accurate estimation of a battery's state of charge (SOC), the prior art has proposed numerous battery condition monitoring systems which rely on indirect indications of battery condition. In broad overview, a lead-acid battery will exhibit different operating characteristics when new as opposed to when used. As the battery deteriorates it will exhibit a higher internal resistance, and will not accept as great an input current. Voltage under load will fall off more rapidly. Indicators related to these factors may be monitored to give an indication of battery condition. However, difficulties arise from the inability to control the conditions of the evaluation.
One such system directed to determining battery condition is U.S. Pat. No. 5,744,963 to Arai et al. Arai teaches a battery residual capacity estimation system. Residual capacity is estimated from a current integration method which utilizes a voltage-current trend calculating section, sensors for obtaining battery current and terminal voltage, a voltage-current straight line calculating section, and a comparator operation for detecting when residual capacity has declined compared to a prior period residual capacity.
Palanisamy, U.S. Pat. No. 5,281,919 describes another method of monitoring a vehicle battery used with a gasoline engine. Five variables are monitored including ambient temperature (T), battery voltage (V), power source (typically an alternator/voltage regulator) voltage (Vs), battery current (I) and time (t). From these variables, the patent provides algorithms for determining the battery's State of Charge (SOC), internal resistance (IR), polarization, and performs various diagnostics.
Palanisamy determines the battery's SOC using a combination of charge integration and open circuit voltage measurements. The open circuit portion of the test relies on a 0.2 voltage drop per cell from a fully charged lead acid cell to a discharged lead acid cell. Open circuit battery voltage (OCV or VOCV) may be taken with the engine on, but is measured at a point in time which avoids effects of polarization of the battery. Open circuit voltage is deemed to coincide with the absence of current flows into or out of the battery for a minimum period. Current integration counts current flow (I) into and out of the battery. Monitoring starts from a point of predetermined charge of the battery, preferably a full charge as determined by the open circuit voltage test. As Palanisamy observes, current integration is subject to error from battery out gassing and deterioration of the physical condition of the battery. The combination of the results is offered as an improvement in measurement of a battery's state of charge, but, due to the systematic errors identified in the patent, is not an necessarily an accurate measurement of the battery's condition.
Internal resistance (IR) is estimated from the open circuit voltage and current flow from the battery following imposition of the starting load. Power output capacity is estimated from IR. Battery polarization arises from non-uniformity of electrolyte density between battery plates and is estimated using Vs, Is and the last battery voltage reading during starting. IR can be used to get battery output capacity for a variety at various temperatures, and then used for a comparison to a table of engine start power requirements supplied by the engine manufacturer.
Palanisamy is limited due to the fact that, under common operating conditions, the current required to crank a gasoline engine is substantially less than the load requirements of a standard load test. Cranking of a gasoline engine usually does not generate data of anywhere near the quality of data produced by controlled condition load test making reference to published voltage charts useless as a mechanism for determining battery condition.
U.S. Pat. No. 6,417,668 to Howard, et al., which is assigned to the assignee of the current application, described an in situ battery monitoring system. Howard provides that upon movement of a vehicle ignition switch from off to on, a process of evaluating the vehicle battery starts. Open circuit voltage and ambient temperature are measured. The open circuit voltage is compared to a table of allowable open circuit voltage ranges as a function of ambient temperature to determine, as an initial matter, if the open circuit voltage is within acceptable ranges for the battery as indicated by manufacturer's specifications. If the open circuit voltage falls within the acceptable range, it is determined if sufficient time has passed since the most recent execution of the routine to avoid polarization effects on the measured open circuit voltage.
If the possibility of polarization effects on the measured open circuit voltage is indicated by a brief lapse since the vehicle battery was last exercised, a load test is imposed on the vehicle battery by engaging an engine starter system to crank the vehicle engine. If the test is automated a safety interlock may be provided based, for example, on whether the hood is open or closed. After a period T, which is preferably fixed in advance, of cranking the engine, voltage across the terminals of the vehicle battery and current from the vehicle battery are measured. Both measurements occur while the battery remains under the load imposed by cranking. A empirically developed specification table indicates battery capacity as a function of the results of the load test. The table may be updated by battery history. An engine required cranking power specification using engine sensor measurements as inputs provides a value for comparison to the capacity figure. A comparison provides an input criterion for generating a displayable result.
Battery modeling provides a partial alternative to empirically generated look up tables. The concept of a battery model using multiple reservoirs with energy flowing between has previously been described. See for example:
1) “Hybrid Vehicle Simulation for a Turbogenerator-Based Power-Train”—C. Leontopoulos, M. R. Etermad, K. R. Pullen, M. U. Lamperth (Proceedings of the I MECH E Part D Journal of Automobile Engineering Volume 212, 1998, Pg 357-368)
2) “Temperature-Dependent Battery Models for High-Power Lithium-Ion Batteries” V. H. Johnson, A. A. Pesaran (Presented at the 17th Electric Vehicle Symposium, Montreal, Canada, Oct. 16-18, 2000)
3) “Battery Characterization System” Thomas J. Dougherty (US Patent application 2004/0212367 A1 Oct. 28, 2004)
4) “Lead Acid Battery Model” (Saber Electronic Simulator, Generic Template Library, October 1999, Synopsys, Inc. 700 East Middlefield RD. Mountain View, Calif.). Both electrical and hydrodynamic analogies have been proposed.
The general model provides an approximation of actual battery characteristics when implemented with modeling and simulation tools, and is useful in the design of electrical systems where batteries are involved. But the models are inadequate for a motor vehicle lead acid battery. The deficiencies have to do with the controlled conditions in design simulations vs. uncontrolled conditions in a vehicle and the need to synchronize in situ monitoring with a real battery.
There are several ways that synchronization can be lost between the model and the target battery. One way is for the initial conditions of the algorithm to be set different from the target. This would occur when the algorithm is initially started/reset, batteries are replaced, etc. Default parameters such as battery state of charge are unlikely to match the real battery in this case. Another loss of synchronization can occur if the device running the algorithm loses power i.e. the vehicle is turned off. Finally, model error also causes loss of synchronization.
Another issue with all in situ monitoring systems is the need for additional equipment and sensors to implement the monitoring and estimation system. With any system it is preferable to minimize hardware modifications to the vehicle as any such equipment will carry a cost and any modification adds to the complexity of the vehicle. Systems that must be mounted onto the vehicles' batteries can also degrade vehicle performance. It is recognized that systems implemented with a minimum of additional equipment must make usable estimations based on less information than otherwise might be available.